Well, it's not a big difference like white VS black for AMOLED screen, but it will certainly do.
The back light of LCD screen is what uses the most power, but to get dark pixels, the LCD must be excited to block the light coming from behind.

AMOLED are better at it because there is no back light, in case of black pixel no light is emitted.
On LCD, you cannot turn off the backlight.
Even if some LED backlit LCD use some techniques to reduce or turing off the intensity on dark/black areas to get deeper black and consume less, but you cannot do that if at least one pixel is not black (IE: text... or anything that is not a pure black/gray).

Originally Posted by campbecw

LCD screens consume more power to produce black than white. If you were using an AMOLED screen, then the "dark" themes would be more energy efficient. But not on the Passport's LCD screen: you're better off using the "light" themes.